Field of the Invention
The invention relates to a method for controlling a magnetic resonance imaging system for the generation of magnetic resonance image data relating to an examination object, wherein magnetic resonance raw data are acquired. The invention also concerns a magnetic resonance system that is operated according to such a method.
Description of the Prior Art
For the acquisition of magnetic resonance raw data, it is known to use multiple pulse sequence segments, each involving one excitation procedure and a subsequent read-out procedure. In the excitation procedure, a first slice selection gradient pulse is generated in the slice selection direction. In addition, an RF excitation pulse is generated, which encompasses N corresponding excitation frequencies for the excitation of N slices that are to be excited simultaneously. In the subsequent read out procedure, a rephasing gradient pulse is generated in the slice selection direction and RF signals for the acquisition of magnetic resonance raw data are received. After receiving RF signals for a preceding pulse sequence segment and before generating the RF excitation pulse for a subsequent pulse sequence segment, a prephasing gradient pulse is generated in the slice selection direction, which pulse is designed such that the gradient moment across all the gradient pulses in the slice selection direction, integrated from the center of an RF excitation pulse to the center of a subsequent RF excitation pulse, has the value 0.
In other words, it is an essential condition for a steady phase of freely precessing spins (steady-state free precession SSFP) that the gradient pulses in the slice selection direction, AS WELL AS and also in the other two directions, are balanced.
Magnetic resonance imaging systems (magnetic resonance tomography systems) are established and proven for a wide range of applications. In this type of image acquisition, a static basic magnetic field B0 that is used for initial alignment and homogenization of the magnetic dipoles nuclei to be studied is superimposed on a rapidly connected magnetic field, known as the gradient field, for local resolution of the image-generating signal. In order to determine the material properties of an examination object that is to be mapped, the dephasing or relaxation time is determined after deflection of the magnetization from the initial direction, such that various material-specific relaxation mechanisms or relaxation times can be identified. Deflection is generally achieved using a number of RF pulses and the local resolution depends on a timed manipulation of the deflected magnetization with the use of the gradient field in a “test sequence” or “control sequence”, which sets a precise timed sequence consisting of RF pulses, modification of the gradient field (by transmitting a switching sequence of gradient pulses), and acquisition of test values.
Typically, a correlation between measured magnetization—from which the aforementioned material properties can be derived—and a local co-ordinate of the measured magnetization in the spatial domain, in which the examination object is arranged, is achieved with the use of an intermediate step. In this intermediate step, acquired magnetic resonance raw data are entered at read-out points in an electronic memory organized as k-space. The co-ordinates of k-space being coded as a function of the gradient field. The contribution made by magnetization (in particular by transverse magnetization, determined in a plane transverse to the aforementioned basic magnetic field) at a particular place on the examination object, can be determined from the data for the read-out point by a Fourier transform that calculates the signal strength of the signal in the local space from a signal strength (contribution made by magnetization) that is assigned to a specific frequency (the local frequency) or phase position.
Magnetic resonance tomography is a relatively slow type of imaging method since the data are recorded sequentially along lines in Fourier space or in k-space, and a certain minimum time is required for the spin relaxation of the excited spins. The method of recording images in two-dimensional slices is clearly less prone to error than recording in three dimensions because the number of coding steps is lower than in a three-dimensional method. Therefore, in many applications, image volumes formed as stacks of two-dimensional slices are used instead of one single three-dimensional image. Even so, the image recording times are very long due to the long relaxation times for the spins, which means, for example, a reduction in comfort for patients who need to be examined. In addition, patients cannot leave the magnetic resonance tomography unit for a short time during the recording of the image or even just change their position since this would ruin the imaging procedure due to the change in position and the whole process would have to start again from the beginning. Consequently, it is an important objective to accelerate the recording of two-dimensional slice stacks.
To accelerate imaging, parallel imaging techniques are used, for example. In some of the imaging techniques, artifacts may occur due to undersampling. These artifacts can be eliminated by using reconstruction algorithms. A further option for the elimination of the artifacts involves the use of CAIPIRINHA (Controlled aliasing in parallel imaging results in higher acceleration, as described in the article BREUER, FELIX A. ET AL., “Controlled aliasing in parallel imaging results in higher acceleration (CAIPIRINHA) for multi-slice imaging”, in: Magnetic Resonance in Medicine, 2005, Vol. 53, No. 3, pp. 684-691, DOI 10.1002/mrm.20401. CAIPIRINHA modifies the artifacts that occur in order to improve the subsequent image reconstruction. CAIPRINHA is therefore superior to some other parallel imaging concepts, in which there is only one subsequent post-processing stage for images impaired by artifacts. In CAIPIRINHA, a number of slices of any thickness and any distance apart are excited simultaneously using multiband RF pulses. The data of the respective slices are then individually sampled, generating images with superimposed slices that are offset from one another. The offset between the slices can be generated by modulation of the phase for the individual slices in the multiband RF pulse.
One technique for rapid image-generation with a high signal-to-noise ratio is TrueFISP (True Fast Imaging with Steady State Precession), also known as Trufi or bSSFP (balanced free precession), as described in OPPELT A. ET AL., “FISP: eine neue schnelle pulse sequence für die Kernspintomographie” [a new rapid pulse sequence for nuclear spin tomography], in: electromedica, Vol. 54, 1986, Issue 1, pp. 15-18. A coherent image recording technique is applied here in which a balanced gradient pulse form is used. A technique is used involving a balanced state in a steady balance with freely precessing spins. TrueFISP functions particularly well with balanced gradient moments and short repetition times TR, it being possible due to the short repetition times to reduce the banding artifacts caused by B0-inhomogenities in the recorded images. Although TrueFISP is a rapid imaging method, there is a need for even faster imaging. For example, real time images can be improved by faster imaging methods. It is also possible to improve the comfort of examination subjects who cannot hold their breath for very long or cannot move. It is therefore a good idea to combine TrueFISP with parallel image sensing, in which multiple slices are scanned simultaneously. However, the repetition times in conventional TrueFISP are so short that gradient activity occurs at all points in time. Therefore, TrueFISP sequences cannot be nested in order to record a plurality of sub-sampled slices simultaneously. The parallel imaging may be limited due to the requirement for the steady state of the freely precessing spins. During the reduction of the phase-coded lines, however, as is the case with CAIPIRINHA, and the recording of slices with a small distance between them, the signal-to-noise-ratio may be significantly worsened. When using the conventional CAIPIRINHA, the robustness of the TrueFISP sequence in withstanding B0-inhomogenitities may be reduced, due to the use of phase-modulated RF pulses with which a number of slices are excited (see STÄB D. ET AL., “Mit CAIPIRINHA beschleunigte Mehrschicht-TrueFISP-MR-Herzperfusionsbildgebung mit vollstandiger Herzabdeckung” [Multilayer TrueFSIP-MR-Heart perfusion imaging accelerated with CAIPIRINHA and covering the heart completely], in: Fortschr Röntgenstr 2009; 181: VO319_6, DOI: 10.1055/s-0029-1221517), as a result of which changes in the signal and contrast and an increased number of banding artifacts may occur.
US 2013/0271128 describes a method with which a simultaneous acquisition of multiple of slices is achieved using the SSFP technique (SSFP=steady state free precession=Trufi), wherein, however, the different phases of the excited spins in the different slices are no longer generated by modulation of the RF pulse, but by variation, of the gradient pulses, that is. To be more precise, the rephasing gradient pulses used within the TrueFISP method and assigned to the individual slices are cyclically modified (for example, their amplitude is modified), such that different phases are imprinted on the freely precessing spins of different slices, without contrast changes occurring due to the phase modulation of the RF pulses that is used in the conventional CAIPIRINHA method. The method described in US 2013/0271128 is also known as multi-slice blipped TrueFISP-CAIPIRINHA (TRUEFISP-CAIPIRINHA with tagged slices). In the particularly effective variant bSSFP (balanced steady state free precession, corresponding to the method described in US 2013/0271128), interference or artifacts may easily occur due to inhomogeneities of magnetic fields and also to eddy currents. In particular, if the simultaneously recorded slices are spaced very close together, an impairment of the image quality occurs in SSFP-SAMS images (SAMS=simultaneous acquisition of multiple slices). For example, a considerable impairment of the image quality occurs if, within the parameters or boundary conditions that are typical of clinical applications, the ratio between the distance between the center of the respective slices and the slice thickness of the relevant slices is lower than 2. The reason for these artifacts is the marked change in the slice prephasers and rephasers that is necessary in order to achieve an adequate phase change between the closely adjacent slices. This marked change leads to more marked eddy currents. In the method disclosed in US 2013/0271128, because the phase of the freely precessing spins is inverted in each read-out step, in parallel with the Trufi-RF phase (Trufi=true fast imaging with steady precession), when the prephasers and rephasers are changed, the eddy current effects are increased and the steady Trufi-state is impaired. This leads to dephasing of the phase-coded spins, resulting in interference and artifacts in the reconstructed image.
Conventionally, the aforementioned problems could be alleviated only by ensuring that a minimum distance was maintained between the individual slices to be scanned. This means, however, that one is restricted to applications involving large gaps between the slices.
The problem therefore exists of achieving simultaneous imaging of parallel adjacent slices with as few artifacts as possible.